Field of the Invention
The present invention relates to a microwave technique, particularly, to a band-pass filter, which can be used in cryo-electronic units at a front end of a receiver used in radio telescopes and satellite communication lines.
Description of the Related Art
Band-pass filters that are installed at input ends of low-noise amplifiers (LNAs) are designed to provide electromagnetic compatibility for radio electronic facilities, i.e., to protect input circuits of a highly sensitive receiver from being affected by electromagnetic radiation outside the operating bandwidth. Currently, transistor LNAs are widely used and replace the previously developed parametric and quantum amplifiers because of their lower noise temperature, broad bandwidth, and operational advantages (stability, efficiency, and ability to operate at cryogenic temperature).
A main characteristic of highly sensitive receivers is the equivalent noise temperature TR of the receiver, which is mainly determined by noise temperature TA of the LNAs and noise temperature TF of passive circuits at the input of the LNAs (for example, a band-pass filter), that is, TR=TF+TA. To decrease the TR, the transistor LNA is cooled to cryogenic temperatures. The noise temperature of the band-pass filter depends on its physical temperature T0 and insertion loss LdB. In the case of the low insertion loss (LdB<0.5 dB), noise temperature of the band pass filter is defined by a simple formula: TF=(LdB/4.34)T0, see Siegman, A. E., Microwave Solid-State Masers, New York-San Francisco-Toronto-London, McGraw-Hill Book Company, 1964. For an element with an insertion loss of 0.1 dB, the noise temperature is 7 K at an operation temperature of 300 K, and the noise temperature is decreased to 1.4K at an operation temperature of 60K. Therefore, the advantage of decreasing the temperature of the front end of the receiver is obvious.
The smaller the insertion loss LdB of the band-pass filter is, the lower the noise temperature TF is. It follows that band-pass filters, made of materials with high conductivity or low value of the microwave surface resistance Rs, have advantage. This is why high-temperature superconductivity (HTS) materials are used in the design of the band-pass filter. Further, the value of the surface resistance Rs of the HTS materials is lower than that of the surface resistance Rs of the conventional metals by several orders, and on the other hand, the HTS materials are in the superconductive state at a cryogenic temperature of or a temperature lower than the temperature of liquid nitrogen (about 77 K), and hence, reliable and economical cryo-coolers can be used in the cryo-electronic unit of the receiver. Currently, the technology of HTS materials has reached a high level such that HTS materials can be used in technical devices. The invention provides a band-pass filter, which uses HTS material in a form of HTS film deposited on the side surface of the dielectric plate (substrate) with low dielectric losses (e.g., superconductive layers of YBaCuO on MgO substrate).
Multi-pole band-pass filters with the so-called E-plane metal insert in a rectangular waveguide are well known. See Vahldieck, R., Bornemann, J., Arndt F., and Graueryolz, D., Optimized Waveguide E-Plane Metal Insert Filters for Millimeter—Wave Applications, IEEE Trans. Microwave Theory Tech., Vol. 31, No. 1, 1983, pp. 65-69. In the E-plane of the rectangular waveguide between the long side walls, a number of metal strips are installed, and regular rectangular waveguides are formed between the metal strips. The regular rectangular waveguides correspond to the resonators in the filter, and the resonators are coupled by means of two portions of the regular rectangular waveguide which are separated by the metal strips. End portions of the rectangular waveguide separated by the metal strips are elements of the filter for coupling with input and output transmission lines.
Fin-line filters, which are band-pass filters based on the principles used in band-pass filters with the E-plane metal insert, are proposed. Instead of an E-plane metal insert, an E-plane dielectric insert is used in the filter. Metal strips of the conventional metal are applied to one or both side surfaces of the insert, see, e.g., Arndt, F., Bornemann, J., Grauneryolz, D., and Vahldieck, R., Theory and Design of Low-Insertion Loss Fin-Line Filters, IEEE Trans. Microwave Theory Tech., Vol. 30, No. 2, 1982, pp. 155-163. Such designs have advantages in the millimeter wavelength range, because the photolithography technology for production which allows maintaining the exact dimensions of metal strips can be used.
The idea of using the inserts from HTS materials instead of the fin-line inserts in the E-plane of band-pass filters has been first expressed in the work of Mansour, R. R. and Zybura, A., Superconductive Millimeter-Wave E-Plane Filters, IEEE Trans. Microwave Theory Tech., Vol. 39, No. 9, 1991, pp. 1588-1492. Experimental study of such a filter has been carried out in the work of Liang Han, Yiyuan Chen, and Yunyi Wang, Design and Performance of Waveguide E-Plane HTSC Insert Filters, 1992 IEEE MTT—S Digest, pp. 913-916. The inventors for the subject application investigated the characteristics of the band-pass filter with an E-plane insert of the HTS material in comparison with the characteristics of the band-pass filter with E-plane inserts of the conventional metal, see Skresanov, V. N., Barannik, A. A., Cherpak, N. T., Y. He, Glamazdin, V. V., Zolotaryov, V. A., Shubny, A. I., Sun L., Wang J., and Wu Y., Experience in Developing Ka-Band Waveguide Filter with HTS E-Plane Insert, the 8th International Kharkov Symposium on Physics and Engineering of Microwaves, Millimeter and Submillimeter Waves (MSMW '2013), Kharkov, Ukraine, Jun. 23-28, 2013. In particular, it has been shown that the advantages of the band-pass filters with the E-plane insert of the HTS material cannot be realized if the problem of providing high-quality contact between the HTS insert and the waveguide walls is not solved. Contact area should have small losses of the microwave power, good thermal contact between the HTS insert and the waveguide walls must be ensured, and the destruction of the fragile substrate plate in cooling-heating cycles of the filter must be prevented.
The closest analogue on the technical essence of the pass-band filter studied by Skresanov et al. is the pass-band filter which comprises a rectangular waveguide of a×b cross-section and a dielectric plate, and on both surfaces of the dielectric plate, high temperature superconductive films are placed with a number of windows. Specifically, the windows are symmetric relative to a dissecting plane of the rectangular waveguide in the height direction, and have the same height, different length, and at different distances relative to each other. The dielectric plate is mounted in an axial plane perpendicular to the long side walls of the waveguide, see Liang Han, Yiyuan Chen, and Yunyi Wang, Design and Performance of Waveguide v E-Plane HTSC Insert Filters, 1992 IEEE MTT—S Digest, pp. 913-916. Lengths of rectangular windows, as well as the distances between the windows, are calculated, and the lengths and the distances are different for different windows. These dimensions determine the Eigen frequencies of the resonators, coefficients of mutual coupling between the resonators and the coupling coefficient of the resonators with the transmission lines, which in turn are determined by the characteristics of the band-pass filter to be achieved. The pass-band filter is a natural development of the known band-pass filters with E-planar fin-line inserts, and can reduce insertion loss due to lower surface resistance Rs of microwave HTS materials compared to conventionally metals. Another technical solution for reducing the insertion loss, which can be combined with the technical solution for reducing the insertion loss by the known band-pass filters with E-planar fin-line inserts, is to redistribute the microwave currents in the waveguide walls by means of the currents in the conductive surfaces of the insert after the dielectric insert is inserted into waveguide.
However, the current band-pass filter has the following technical disadvantages. One of the components of the insertion loss, i.e, the scattering of the microwave power in the contact area between the HTS films and the waveguide walls, should be smaller as compared with the heat Joule loss in the HTS films. In the current band-pass filters, the requirement of the scattering of the microwave power being smaller than the heat Joule loss can be achieved if the surface of the filter housing is polished and thus mechanically in close contact with the HTS films of the insert. The dielectric plate (substrate) should be made of materials with low dielectric losses and a crystal lattice close to the crystal structure of the HTS film. Some single crystal dielectric plates, such as MgO, LaAlO3, and Al2O3, have the properties of low dielectric losses and crystal lattices close to the crystal structure of the HTS film, however, the dielectric plates are fragile and may be easily damaged in the cooling-heating cycles of the filter due to close mechanical contact with the filter body. The technical problem of the dielectric plate being easily damaged still cannot be solved, even if the filter body is made of a material with a coefficient of a linear expansion close to that of the dielectric plates, (for example, the filter body is made of titanium, while the filter body is made of MgO). The reason is that temperature gradients that are caused during the cooling in the filter body introduce unacceptable mechanical stresses in the dielectric plate.